It is known that "pumping" is a process by which the energy of molecules in a gas medium is raised to a vibrational and rotational energy level to allow lasing to occur. The total number of excited molecules per cm.sup.3 above a known threshold for a given medium creates a "population inversion" which allows the excited molecules to lase, i.e., emit photons, when excited molecules transition from one excited (pumped) energy level to a lower level.
It is also known that there are numerous ways to pump molecular gas lasers. For example, a Carbon Dioxide (CO.sub.2) gas laser may be pumped electrically by injecting electrons into the CO.sub.2 by a plasma and the collisions between the electrons and molecules excite the gas molecules. This technique allows the same CO.sub.2 molecules to be re-used each time the laser is pumped. Helium-Neon (HeNe) lasers are also pumped in this fashion. However, electrically pumped CO.sub.2 -lasers typically have a low efficiency, e.g., 5-10%, defined as output optical power at the desired wavelength divided by total electric input power. Also, for high power CO.sub.2 lasers, the gas is typically pumped through the cavity in a "closed cycle" to dissipate heat. This is also called a "flowing" laser system.
Some gas lasers, such as Hydrogen Fluoride (HF) and Deuterium Fluoride (DF), may not be pumped electrically because direct collisions with electrons will not excite the gas molecules to allow lasing to occur. Instead, pumping is performed chemically by reacting two or more atoms/molecules together. For example, for an HF laser, Fluorine (F) atoms are reacted with H.sub.2 to create a vibrationally excited HF. With cw pumping, the reactants are consumed in the reaction, thereby requiring a fresh supply of reactants to re-excite (or re-pump) the system. Thus, an HF laser is a "consumable" or "open cycle" laser system that does not allow reuse of the same molecules. Also, consumable lasers are expensive to run because chemicals must be continuously provided and pumped away. Further, both consumable and flowing systems take up more physical space because they require pump hardware along with nozzles, valves, and other plumbing hardware.
It is also known that gas lasers, such as HF, can sustain high laser output power when they are chemically pumped, and have a highly coherent output beam.
Alternatively, as is known, a gas laser may be thermally pumped by heating the gas to a predetermined temperature, e.g., a gas dynamic CO.sub.2 laser. However, this process also has a low efficiency and requires external heating hardware or a chemical reaction to generate the required gas temperature.
Furthermore, when a gas laser is pumped by heat or electrons, etc., all energy levels are pumped. Consequently, all the excitable levels within the medium are pumped and lasing can occur from all potential lasing levels. The energy contained within the nonlasing levels is released as waste heat which must be removed by rapid flowing of the gas. For HF or DF gases, to permit amplification of a single or small number of output lasing wavelengths (i.e., a narrow wavelength spread), a laser cavity must be designed to reflect (and partially transmit) the desired output lasing wavelengths, and reflect the undesired lasing wavelengths with sufficient reflection loss to prevent light amplification from occurring, e.g., by using a grating for wavelength discrimination. This type of pumping causes inefficiencies because the total energy required to pump the gas to the excited state (i.e., the sum of all the individual pumping energies needed to provide each lasing wavelength) is much greater than the desired optical energy output (i.e., a single or small number of output wavelengths). Thus, the energy expended to pump (excite) the unused lasing transitions is lost.
Therefore, it would be desirable to obtain a gas laser that does not consume gas for lasing, does not require fast flowing of gas for cooling, and does not have the lasing inefficiencies of currently available pumped gas lasers, yet provides the highly coherent output beam quality of gas lasers.